enable using ephemeral disks by default (#461)

src/api-service/__app__/onefuzzlib/autoscale.py

@@ -75,6 +75,7 @@ def scale_up(pool: Pool, scalesets: List[Scaleset], nodes_needed: int) -> None:
             region=autoscale_config.region,
             size=max_nodes_scaleset,
             spot_instances=autoscale_config.spot_instances,
+            ephemeral_os_disks=autoscale_config.ephemeral_os_disks,
             tags={"pool": pool.name},
         )
         nodes_needed -= max_nodes_scaleset

src/api-service/__app__/onefuzzlib/azure/vmss.py

@@ -208,6 +208,7 @@ def create_vmss(
     image: str,
     network_id: str,
     spot_instances: bool,
+    ephemeral_os_disks: bool,
     extensions: List[Any],
     password: str,
     ssh_public_key: str,
@@ -259,7 +260,9 @@ def create_vmss(
         },
         "virtual_machine_profile": {
             "priority": "Regular",
-            "storage_profile": {"image_reference": image_ref},
+            "storage_profile": {
+                "image_reference": image_ref,
+            },
             "os_profile": {
                 "computer_name_prefix": "node",
                 "admin_username": "onefuzz",
@@ -300,6 +303,13 @@ def create_vmss(
             },
         }
 
+    if ephemeral_os_disks:
+        params["virtual_machine_profile"]["storage_profile"]["os_disk"] = {
+            "diffDiskSettings": {"option": "Local"},
+            "caching": "ReadOnly",
+            "createOption": "FromImage",
+        }
+
     if spot_instances:
         # Setting max price to -1 means it won't be evicted because of
         # price.

src/api-service/__app__/onefuzzlib/workers/scalesets.py

@@ -82,6 +82,7 @@ class Scaleset(BASE_SCALESET, ORMMixin):
         region: Region,
         size: int,
         spot_instances: bool,
+        ephemeral_os_disks: bool,
         tags: Dict[str, str],
         client_id: Optional[UUID] = None,
         client_object_id: Optional[UUID] = None,
@@ -93,6 +94,7 @@ class Scaleset(BASE_SCALESET, ORMMixin):
             region=region,
             size=size,
             spot_instances=spot_instances,
+            ephemeral_os_disks=ephemeral_os_disks,
             auth=build_auth(),
             client_id=client_id,
             client_object_id=client_object_id,
@@ -237,6 +239,7 @@ class Scaleset(BASE_SCALESET, ORMMixin):
                 self.image,
                 network_id,
                 self.spot_instances,
+                self.ephemeral_os_disks,
                 extensions,
                 self.auth.password,
                 self.auth.public_key,